1. Field of the Invention
The present invention relates to a receiving side filter of a duplexer applied to, for example, a portable terminal device and the like.
2. Description of the Related Art
In recent years, requirement for an elastic wave duplexer having a balanced output as a receiving side output has been increasing. The reason is that a circuit using a balanced input has been increasingly used as a processing circuit of reception signal provided at a rear stage of receiving side filter. In such a case, it is common that a ladder-type filter in which a several stages of elastic wave resonators are connected in a serial-parallel manner and a longitudinal mode resonator type filter having an unbalanced-balanced conversion function are respectively used in a transmitting side circuit and in a receiving side circuit as circuits of elastic wave duplexer. FIG. 13 and FIG. 14 show an example of a conventional duplexer, in which 5 denotes an elastic wave resonator, 10 denotes a piezoelectric substrate, 11 denotes a ladder-type filter (transmitting side filter), 12 denotes a receiving side filter in which, for example, two balance-connected longitudinal mode resonator type filters 12a are connected, 13 denotes a phase shifter, 14 denotes an input/output port, 15 denotes a transmission input port, and 16 denotes a balanced output port.
In order to preferably maintain an isolation characteristic between the filters 11 and 12 in such a duplexer, a measure such as to increase a physical distance between the filters 11 and 12, to divide the filters 11, 12 as individual chips, or to additionally provide a metal shield 100 between the filters 11 and 12 as shown in FIG. 15, has to be taken. However, these methods have problems such that a size of device becomes large and the number of manufacturing steps and cost are increased in accordance with the half-division of the filters 11, 12, and besides, there is a disadvantage that a desired isolation characteristic cannot be obtained even if the above measures are taken. As will be shown in later-described FIG. 6, in the duplexer in FIG. 14, an increase in loss ascribable to an electromagnetic coupling between the filters 11 and 12 occurs in a transmission band in the vicinity of 830 MHz, and only 54 dB of attenuation amount is obtained as an effective value, so that the duplexer lacks performance as a duplexer for portable terminal device in recent years.
Although Patent Document 1 discloses a SAW filter using a surface wave, no study has been made regarding the aforementioned problems.    [Patent Document 1] Japanese Patent Application Laid-open No. 2003-209456 (FIG. 1, FIG. 8)